Translation:Oh my God!
Each noun conjugation forms vocative differently. Some of them are the same as the nominative, some of them are not (masculine nouns are the most common to have different vocative). Intonation changes, too. For the phrase above it is common to use Ω Θεέ, and when adressing someone who is far away you can use (informally) ε (or ω ρε) as a pseydo-article and the vocative case.
Vocative is used whenever you adress somebody. Many of my friends think that their Greek male friends have names like Κώστα, Γεώργο, Πέτρο because that is what they are called when you talk to them
Κλητική/ Vocative is used when you speak or write to somebody
Vocative plural is nominative plural for all genders and vocative sing. is equal to nominative sing for feminines and neuters: κύριες! αγάπη μου! παιδί μου! Μαρία!
In singular the masculines loose their -ς and -ος becomes -ε for all nouns except names with two syllables: Ανδρέα! Λαέ! Θεέ! Κωνσταντίνε! but Τάσο!